The immune system of elderly humans can exhibit symptoms of deficiency or dysregulation. The long-term objective of this project is to understand how these phenomena relate to or are caused by alterations in the mature CD4+ T-cell pool. The involution of the thymus (the source of new CD4+ T-cells) in conjunction with a lifetime of antigen (Ag)-induced cell differentiation result in marked changes in this cell population, including a gradual shift from a prevalence of naive, Ag-inexperienced cell subsets in early life to a predominance of differentiated memory or effector cell subsets in late life. We propose to use the mouse model and in vitro methods to study the impact of age-related shifts in cell subset frequencies on gene programs related to cell cycle, cell death, cell differentiation, and effector cell function. First, we will test for age-related changes in the inducible expression of cytokines and cytokine receptors by isolated naive and memory cell subsets, and will determine whether aging affects the cell activation requirements of these subsets. To discern whether further differentiative change occurs within the memory cell group during aging, we will attempt to identify, enumerate, and test the responsiveness of Th0-, Th1-, and Th2-like memory cells. In a second series of studies, naive and memory CD4+ cells will be assessed for age-related changes in the patterns of inducible cell cycle entry and progression and sustained clonal growth. In addition, the CD4+ cell subsets will be examined for age-related differences in their programs for autonomous cell death. The relationship of these programs to patterns of anti- or pro-death gene expression will also be addressed. Lastly, we will use an in vitro model for cytokine-driven cell differentiation to study possible age-related changes in the capacity of naive and memory cell subsets to develop into Th0-, Th1- or Th2-like cells; to determine whether these differentiative processes remain flexible in late life; and to examine the influences of late-life memory cells on the differentiation of co-localized naive cells. Results from these studies should provide important information on age-related changes in the representation and function of distinct CD4+ cell subsets, and should yield insight into possible mechanisms of immune deficiency of dysregulation related to altered programs for clonal expansion, apoptosis, cell differentiation, and effector function in this cell group. This information should contribute to the rational design of geriatric vaccines to target naive (Ag new to the host) or memory (previously-encountered Ag) CD4+ cells.